The invention relates to testing the system for protecting a turbomachine against overspeed, the test being performed during starting of the engine. The invention is applicable to aeroengines and to industrial turbines.
An excessive speed of rotation in a turbine engine, known as overspeed, can have particularly severe consequences, in particular it can lead to turbine rotor disks bursting, with destructive effects on the engine.
An engine is therefore generally fitted with a system for protecting it against overspeed.
In usual manner, such a system includes a cutoff valve inserted in the circuit for feeding the combustion chamber of the engine with fuel. Overspeed is detected by an electronic overspeed protection unit when the speed of rotation of a turbine shaft exceeds a limit value or overspeed threshold. When overspeed is detected, the electronic overspeed protection unit sends a command to close the cutoff valve or to reduce its flow section via various components of electronic, electrohydraulic, or hydraulic type. Together with the cutoff valve, these various components form parts of the overspeed protection system.
It is known to incorporate a test of the overspeed protection system in the procedure for automatically starting an engine. Reference may be made to application EP 1 036 917 A1. That describes a process whereby, once the engine has begun to rotate on being driven by the starter, the speed threshold is given a test value. The test value is selected to be substantially below the ignition speed, i.e. the speed of rotation at which fuel is injected into the combustion chamber. When the speed of rotation exceeds the test overspeed threshold, it is verified that the overspeed protection system has operated properly. If so, then when the ignition speed is reached, the overspeed threshold is switched from its low test value to a real value.
In that known test process, verification is performed by detecting the position of the core of a fuel metering unit that is taken to its minimum position by a hydraulic device controlled by the electronic regulation system of the engine and that, in this minimum position, causes the cutoff valve to be closed. There is thus no actual verification that the cutoff valve reaches its closed position. In addition, the effectiveness of the overspeed protection function after testing assumes that the changeover of the overspeed threshold from the low, test value to the real value takes place correctly.